An RFL adhesive is mainly used as an adhesive for bonding tire cords to rubber in tires. The RFL adhesive is a mixture of a resorcinol-formaldehyde resin and a rubber latex. In the early stage, a natural rubber latex was used as the rubber latex, but later superseded by synthetic rubber latices. Among them, a latex of a VSB copolymer (VSB latex) is known to produce an RFL adhesive having the best performance. In addition to the RFL adhesive, the VSB latex can also be used as an adhesive in the form of a mixture with an isocyanate, or with an isocyanate and an ethyleneurea compound.
The problem still desired to be improved in the aforesaid adhesives based on a VSB copolymer, particularly an RFL adhesive based on a VSB copolymer is that the adhesion strengths of these adhesives decrease with time. For example, a reduction in adhesion strength between tire cords and rubber occurs during the running of an automobile, and may sometimes result in the breakage of the tire. Although the cause of the decrease of the adhesion strength in a tire has not yet been completely elucidated because of the great complexity of actions exerted on the tire, it is believed to be at least certain that the temperature rise of the tire owing to internal heat buildup is one cause. During the running of an automobile, the tires undergo repetitive stresses of various types such as tensile, shearing, compressive and flexural stresses, and the resulting hysteresis loss (i.e., energy loss) causes heat buildup within the tires.
The present inventor performed an experiment simulating the temperature rise of a tire owing to internal heat buildup (the experiment will be described in detail hereinbelow), and found that the VSB copolymer has extremely poor resistance to high temperatures, and when heated to 160.degree. C., it decomposes to evolve gases and consequently deforms (Experiment A). When sulfur is added to the VSB copolymer, the generation of gases and, consequently, the deformation of the VSB copolymer become remarkable as a result of heating (Experiment B). The present inventor found that the addition of a small amount of a certain amino acid to the VSB copolymer markedly reduces the generation of gases and the deformation of the copolymer owing to heating (Experiment C). The present inventor also found that when sulfur is added to the VSB copolymer containing the amino acid produced in Experiment C, the generation of gases and the deformation of the copolymer owing to heating are further reduced (Experiment D). In other words, when the VSB copolymer alone is used, the addition of sulfur accelerates the generation of gases and the deformation of the copolymer, but when the amino acid is present together with the VSB copolymer, the addition of sulfur unexpectedly inhibits the generation of gases and the deformation of the copolymer.
As shown by these Experiments, the thermal stability of the VSB copolymer can be improved by adding a specified amino acid to the VSB copolymer. This is desirable for removing the decrease with time of the adhesion strength of adhesives based on the VSB copolymer, especially the RFL adhesive.